Copper-catalyzed direct α-ketoesterification of propiophenones with acetophenones via C(sp3)-H oxidative cross-coupling

Article abstract of DOI:10.1039/c4cc09524c

A novel copper-catalyzed direct α-ketoesterification of propiophenones with acetophenones via C(sp³)-H oxidative cross-coupling was developed. The reaction utilized O₂ as a clean oxidant with high atom economy and the starting materials are facile and commercially available.

Full text of DOI:10.1039/c4cc09524c

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COMMUNICATION
Copper-catalyzed direct α-ketoesterification of
propiophenones with acetophenones via C(sp3)−H
oxidative cross-coupling
Cite this: DOI: 10.1039/x0xx00000x
Juan Du,a Xiuli Zhang*,a,c Xi Suna and Lei Wang*,b,c
Received 00th xxxxx 2014,
Accepted 00th xxxxx 2014
DOI: 10.1039/x0xx00000x
www.rsc.org/
Previous work:
A
novel copper-catalyzed direct α-ketoesterification of
O
O
O
propiophenones with acetophenones via C(sp3)−H oxidative
cross-coupling was developed. The reaction utilized O2 as a clean
oxidant with high atom economy and the starting materials are
facile and commercial available.
m-CPBA/PhI
or H2O2/Ac2O
O
R2
eq. 1
eq. 2
R1
+
R1
OH
O
R2
H
O
O
O
m-CPBA/PhI
O
O
R
OH
n = 1 or 2
n
R
or H2O2/TBAI
n
H
α-Functionalization of ketones is a significant and fundamental
organic reaction.1 The direct cross-coupling at α-C(sp3)−H of
arylketons is one of the most concise and effective route to them.
Among the cross-coupling reactions, α-allylation,2 α-alkylation,3 α-
amination,4 α-arylation,5 α-halogenation,6 α-hydroxylation,7 α-
oximation,8 α-azidation,9 α-imidation,10 α-vinylation11 of arylketones
have received much more attentions, and the transition-metal as a
catalyst is essential in the most of cases. The transition metal used in
the reactions was found to be Pd,2a,2b,5c-5i,7b Cu,3a,4,11a,11c Rh,3b,3c
Ru,3d,3e Ni,5b or Ir.3f On the other hand, the oxidative cross-couplings
of arylketons have been developed in recent years. For example, the
synthesis of α-ketoamides from acetophenones with secondary
amines12 and dialkylformamides13 in the presence of TBHP as an
oxidant was reported.
α-Acyloxycarbonyl compounds play an essential role in biological
processes, serve as the backbones in some natural products, and are
used as significant building blocks in organic synthesis.
Traditionally, they can be prepared by the reaction of α-halocarbonyl
compounds with carboxylates14 or the direct oxidative coupling of
carbonyl compounds with Pb(OAc)4, Tl(OAc)3, and Hg(OAc)2.15
Most recently, hypervalent iodine compounds as effective oxidants
to promoted esterification were investigatived.16 For example, the
intra- and intermolecular oxidative couplings of ketones with
carboxylic acid catalyzed by hypervalent (diacyloxyiodo)benzene
generated in situ from iodobenzene and meta-chloroperbenzoic acid
(m-CPBA) were demonstrated (Scheme 1, Eq. 1 and 2).16b,16d The
same reaction was also mediated by H2O2 and Ac2O as oxidant (Eq.
O
O
O
R2
R
O
TBHP/TBAI
R1
+
eq. 3
eq. 4
R1
OH
R
O
R2
H
O
O
R2
Ar
O
TBHP/TBAI
R1
+
Ar
OH
R1
R2
O
H
This work:
R1
O
CuI (20 mol%)
HOAc (1 equiv)
DMSO, 120 oC
O2
O
O
R1
Ar
+
O
Ar
H
O
O
Scheme 1 Direct α-ketoesterification of ketones
1),17 as well as by H2O2/Bu4NI and TBHP/Bu4NI (Eq. 2 and 3).18
Moreover, α-acyloxycarbonyl compounds could be prepared by the
Bu4NI-catalyzed reaction of ketones with benzylic alcohols in the
presence of TBHP (Scheme 1, Eq. 4).19
In this paper, an efficient Cu-catalyzed C(sp3)−H oxidative
cross-coupling of propiophenones with acetophenones was
developed. The reaction underwent smoothly in one-pot with good
regioselectivity, and molecular oxygen as oxidant is inexpensive,20
abundant and environmentally friendly (Scheme 1).
At the investigation of the reaction conditions, propiophenone
(1a) and acetophenone (2a) were chosen as the model substrates.
The results were shown in Table 1. Firstly, the effect of oxidant on
the model reaction was examined. When the reaction was performed
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Table 1 Optimization of the reaction conditionsa
Table 2 Scope of the substratesa
Entry
1
Catalyst
CuI
Oxidant
O2
Additive
Solvent
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
NMP
Yieldb(%)
-
20
2
CuI
TBHP
H2O2
DTBP
O2
-
trace
11
3
CuI
-
4
CuI
-
N.R.
18
5
CuBr
CuCl
CuBr2
CuCl2
CuO
Cu(OAc)2
Cu(OTf)2
CuI
-
6
O2
-
N.R.
N.R.
trace
N.R.
N.R.
N.R.
trace
N.R.
N.R.
N.R.
N.R.
N.R.
40
7
O2
-
8
O2
-
9
O2
-
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
O2
-
O2
-
O2
-
-
CuI
O2
DMF
CuI
O2
-
THF
CuI
O2
-
toluene
CH3CN
DCE
CuI
O2
-
CuI
O2
-
CuI
O2
HOAc
PivOH
TFA
CF3SO3H
PhCO2H
HOAc
Cs2CO3
Et3N
Pyridine
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
CuI
O2
37
CuI
O2
39
CuI
O2
31
CuI
O2
36
a
Reaction conditions: 1 (0.375 mmol) and 2 (0.25 mmol), CuI (0.05
mmol, 0.20 equiv), HOAc (0.25 mmol, 1.0 equiv), O2 (1.0 atm), DMSO
(0.30 mL), at 120 oC for 12 h. b Isolated yields.
CuI
O2
63c
CuI
O2
N.R.
N.R.
N.R.
CuI
O2
detected (Table 1, entry 12), and the reaction failed in DMF (N,N-
dimethylformamide), THF, toluene, CH3CN, or DCE (1,2-
dichloroethane) (Table 1, entries 13−17). To our delight, additional
an organic acid, such as HOAc, PivOH, TFA (trifluoroacetic acid),
CF3SO3H, or PhCO2H enhanced the reaction, and HOAc was found
to be most effective one (Table 1, entries 18−22). However,
inorganic and organic base including Cs2CO3, Et3N, pyridine,
completely failed (Table 1, entries 24−26).
With optimized reaction conditions in hand, a variety of
propiophenones and acetophenones were examined to illustrate the
efficiency and scope of the oxidative cross-coupling (Table 2).
Acetophenones with an electron-donating group (Me, Et) and two
methyl groups on the benzene rings reacted with propiophenone to
generate the desired products in 51−58% yields (Table 2, 3b−3g). It
should be noted that acetophenone with acetal group generated 3hin
61% yield, and acetophenone with two methoxy groups afforded 3i
in 48% yield. On the other hand, acetophenones with an electron-
withdrawing group (Cl, Br, I) on the benzene rings reacted with
propiophenone to generate the desired products (3j−3l) in 42−45%
yields. When acetophenones with more electron-deficient groups
CuI
O2
a Reaction conditions: propiophenone (1a, 0.375 mmol) and acetophenone
(2a, 0.25 mmol), catalyst (0.05 mmol, 20 mol%), oxidant (4.0 equiv, 1.0
mmol), additive (1.0 equiv, 0.25 mmol), solvent (2.0 mL), at 120 oC for 12 h.
b Isolated yields. c DMSO (0.3 mL). N.R. = No reaction.
in the presence of CuI in DMSO (dimethyl sulfoxide) under oxygen
atmosphere at 120 °C for 12 h, the product 3a was obtained in 20%
1
yield (Table 1, entry 1), which was characterized by HRMS, H and
13C NMR, and confirmed by single crystal X-ray crystallography.21
However, additional peroxide, such as TBHP (tert-butyl
hydroperoxide, 70% aqueous solution), H2O2 (30% aqueous
solution) or DTBP (di-tert-butyl peroxide) was added to the reaction,
providing less yields of 3a (Table 1, entries 2−4). Next, the effect of
Cu-catalyst was examined. CuBr gave the comparable result with
CuI, and other Cu-catalysts including CuCl, CuBr2, CuCl2, CuO,
Cu(OAc)2, and Cu(OTf)2 shut down the reaction completely (Table
1, entries 5−11). The solvent also plays an important role in the
reaction. When the reaction was carried out in NMP (N-
methylpyrrolidone), only trace amount of the desired product was
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(CO2CH3 and CF3) reacted with propiophenone, no desired products
were detected. Under the optimized conditions, 1-acetyl naphthalene
gave the desired product (3m) in 60% yield. A slight steric effect
was observed during the formation of 3b and 3f. A variety of
propiophenones were also examined and the results were shown in
Table 2. Propiophenones with an electron-rich (Me, MeO) or
electron-deficient group (Cl) on the phenyl rings reacted with 4-
methylacetophenone and 4-fluoroacetophenone to give the desired
products (3n−3s) in 44−59% yields. It should be noted that
butyrophenone underwent the reaction with acetophenone to afford
the corresponding products 3t in 53% yield.
In order to gain insight into the mechanism, several control
experiments were performed. When a radical scavenger (2,2,6,6-
tetramethylpiperidyl-1-oxyl, TEMPO, 1.0 equiv) was added to the
system, the reaction was completely shut down. It indicated a radical
pathway may be involved in the reaction. In the presence of
CuI/HOAc/O2, the reaction of acetophenone (2a) generated 2-oxo-2-
phenylacetic acid (4a) in 32% yield (Scheme 2, Eq. 1), and
propiophenone (1a) afforded α-iodo-propiophenone (5a) in 11%
yield (Scheme 2, Eq. 2). The prepared 5a reacted with 4a in DMSO
at 120 oC to afford 3a in 71% yield (Scheme 2, Eq. 3).
Scheme 2 The control experiments
Table
propiophenones (1)a
3
The reaction of 2-oxo-2-arylacetic acids (4) with
O
O
O
R1
Ar
CuI (20 mol%)
DMSO, 120 oC
Air
OH
R1
O
+
Ar
H
O
O
O
1
3 b
4
In order to confirm the activity of α-ketoacids, the reaction of 2-
oxo-2-arylacetic acids (4) with propiophenones (1) was investigated,
shown in Table 3. As anticipated, all of the corresponding
compounds were obtained in good to excellent yields. Electron-
donating groups (Me, MeO, Et, acetal) or electron-withdrawing
groups (F, Cl, Br, I) on benzyl rings of ketoacids gave the superior
yields to the corresponding acetophenones (3a−3l and 3n−3s, Table
3 vs Table 2). 2-(Naphthalen-1-yl)-2-oxoacetic acid reacted with 1a
to generate the corresponding product 3m in 76% yield (Table 3). It
is important to note that the reactions of 2-(furan-2-yl)-2-oxoacetic
a
Reaction conditions: 1 (0.375 mmol) and 4 (0.25 mmol), CuI (0.05
mmol, 0.20 equiv), DMSO (0.30 mL), open flask at 120 oC for 12 h.
Isolated yields.
b
acid
with
1-(4-methylphenyl)propan-1-one,
1-(4-
methoxyphenyl)propan-1-one and 1-(4-chlorophenyl)propan-1-one
afforded the desired products (3u, 3v and 3w, Table 3) in 92, 90 and
91% yields, respectively. However, no desired product was found for
the reactions of 2-acetylfuran with propiophenone.
Based on the previous reports and the above results, a possible
reaction mechanism was proposed in Scheme 3. Initially, iodide
anion was oxidized by molecular oxygen or CuI/O2 to iodine radical,
which reacted with propiophenone (1a) to generate α-carbonyl
radical I.4 The obtained I lost an electron in the presence of CuII
from CuI/O2 to generate α-carbonyl cation II,4,22 which reacted with
iodide ion to form intermediate 5a. On the other hand, acetophenone
(2a) reacted with molecular iodine, which produced from the
reaction of iodide anion and O2 or CuI/O2, to afford 2-iodo-1-
phenylethanone, as an intermediate III.22a The formed III was
oxidized with DMSO to phenylglyoxal IV, and was further oxidized
by O2 to intermediate 2-oxo-2-phenylacetic acid (4a).23 Finally, the
reaction of 4a with II or 5a provided the desired product 3a. In order
to further investigate the mechanism, the trapping of free radical
intermediate with TEMPO by HPLC-HRMS probe was carried out.
The coupling product of α-carbonyl radical I with TEMPO was
confirmed by HRMS (ESI for detail). The good regioselectivity in
the reaction is considered as the more stable of α-carbonyl radical I
from propiophenone (1a) compared with the corresponding α-
carbonyl radical from acetophenone (2a).
Scheme 3 The proposed mechanism
In summary, we have developed a novel and efficient CuI-
catalyzed direct α-ketoesterification of propiophenones with
acetophenones via C(sp3)−H oxidative cross-coupling reactions. The
reactions underwent smoothly in one-pot with good regioselectivity.
The molecular oxygen is employed as an oxidant. Meanwhile, the
desired products were obtained in superior yields from the reactions
of propiophenones with 2-oxo-2-arylacetic acids, which were from
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mechanism are underway currently.
This work was financially supported by the National Science
Foundation of China (No. 21172092), and the Department of
Education, Anhui Province (No. KJ2013A122).
8.
9.
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